There's a white bishop on d3 at the end of move 15. Aronian's exploration is departing from what was actually played following move 15 because on move 16 in the game, White's bishop retreated back to e2.

@blueemu - after 19...Ndf2+ White has 20.Kh2, preventing the discovered check and removing the mate threat.

PS in this video , posted shortly after the match yesterday, kingscrusher plays live through many variations using Houdini - not an in-depth analysis, but it brings to light the dozens of possibly lethal combinations which arose after a lot of (for the likes of us) seemingly logical moves - very worth watching!

The move 20. Rxf2 was also possible after the other Knight check (... Ngf2+), leading to an identical position... but Houdini rejected it in favor of 20. Kh2 instead. That means that (in Houdini's opinion) 20. Kh2 must be less disadvantageous for White than returning the Rook with 20. Rxf2.

So why not FORCE White to return the Rook, by checking with the d-Knight instead of with the g-Knight? It forces White into the same line that Houdini had already rejected as inferior.

So why not FORCE White to return the Rook, by checking with the d-Knight instead of with the g-Knight? It forces White into the same line that Houdini had already rejected as inferior.

... is there a flaw in my reasoning?

No, you´re quite correct! I made the mistake - I´d tried so many lines that I´d copied the wrong one into the analysis board here for posting. Houdinis suggestion is in fact also yours; checking with the d-knight is correct. Sorry about that! Here´s the line: